Analog current loops, such as 4-20 mA current loops or older 10-50 mA current loops, are commonly used to transmit signals (e.g., measured sensor values, control values, etc.) to process control devices, such as actuator valves, motor speed controls, panel meters, etc. In a typical scenario, a sensor or controller provides an output voltage representing an indicated value such as a measured value (e.g., temperature, pressure, etc.) or setpoint, respectively, and a transmitter converts the output voltage to a proportional 4-20 mA current in a closed current loop. Subsequently, a process control device receives the 4-20 mA current signal and processes the signal via an internal resistance, which can be easily measured by an analog input of the process control device.
Some process control devices that receive signals from current loops rely on power from the current loop itself, but other devices receive power from separate operating sources. In the past, process control devices, such as field instruments located on remote oil or gas structures, relied on pneumatic power. In such cases, the instruments could operate with only a small current source (4-20 mA) for signaling purposes, while other components of the instruments were pneumatically powered.
In recent years, however, a large number of field instruments or process control devices rely on an operating electric power source separate from a 4-20 mA current loop. For example, some valve actuators rely on a voltage input, such as +10V and −10V, to operate, even though the actuators receive control signals via a 4-20 mA current loop. As such, a situation can arise in which a 4-20 mA current is applied to an instrument while certain components are not powered by the corresponding operating voltage source, such as during a power failure. This situation often results in damage to sensitive electronic components (e.g., integrated circuits).